Sulphonated polyarylethersulphone and polyaryletherketone polymers are useful membrane materials and have been used in filtration processes such as reverse osmosis and nanofiltration. Sulphonated poly(arylene ether sulphone)s have been produced by post-polymerisation sulphonation of commercially available bisphenol-A based poly(ether sulphone):
U.S. Pat. No. 5,693,740 discloses sulphonated polyarylethersulphone copolymers of the formula (I):

Similar polymers have been disclosed for use as membranes in polymer electrolyte membrane (PEM) fuel cells. The most commonly used polymers in polymer electrolyte membranes are perfluorinated sulphonic acid polymers such as Nafion® and Flemion®. However, the perfluorinated polymers are expensive and have limited ionic conductivity at elevated temperatures (greater than 100° C.). Sulphonated polyarylethersulphone and polyaryletherketone polymers have been investigated as alternative polymer electrolyte membrane materials.
U.S. Pat. No. 5,985,477 discloses a polymer electrolyte comprising a sulphonated copolymer which is obtained by sulphonating a copolymer formed from three monomers:
WO 02/25764 discloses a sulphonated copolymer as shown below:
This copolymer is said to provide a polymer electrolyte membrane with improved thermal stability and improved protonic conductivity.